The present invention relates generally to implantable medical devices for treating cardiac dysrhythmias, and more particularly to a multi-mode device which is adapted to provide bi-ventricular therapy to the patient""s heart in response to sensing applicable dysrhythmias.
Progress in medicine is based largely on progress in the technology of devices and apparatus for administering therapy. For example, significant advances in design techniques that have resulted in continuing reductions in the size of implantable defibrillators, including size of the function generator itself as well as in the heart leads associated therewith, have led to a capability to implant defibrillators at considerably lower risk to patients. During the first few years following the advent of implantable defibrillators, implant procedures required general anesthesia and thoracotomy, and the patient was faced with all of the risks associated with opening the chest cavity. The mortality rate of the procedure tended to limit widespread use of the device.
In recent years, with lower defibrillation thresholds (DFTs) and reduction in high voltage capacitor and battery sizes, smaller and more easily implantable devices have been developed, which have allowed this operation to be performed today under only local anesthesia. Smaller diameter and more easily inserted transvenous lead systems have overcome the need for a thoracotomy, and mortality associated with the procedure has been concomitantly reduced to less than one percent. The cosmetic aspects of such an implantation have also improved, with device size and weight allowing it to be implanted in the pectoral region that had previously been reserved for devices capable of only pacing functions, rather than the lower abdomen.
Nevertheless, at least two issues remain to be resolved with respect to present-day implantable defibrillators. For one thing, despite size reduction owing to the aforementioned advances in technology, the devices are still relatively large. At present, the limitations on size reduction are primarily attributable to the magnitude of energy required to achieve successful defibrillation with an adequate safety margin. A capacity for energy delivery of 25 to 32 joules (J), on average, currently remains the standard for implantable defibrillators. This minimum energy requirement mandates production and use of devices ranging from 40 to 50 cubic centimeters (cc) in volume and 80 to 100 grams (g) in weight.
Another issue that remains to be resolved is the provision of a continuously uniform, homogeneous electric field distribution during application of the relatively high energy defibrillating shocks to the heart. Studies performed on animals and humans indicate that to achieve a successful defibrillation with a lowered energy content shock requires a substantially uniform electric field distribution throughout the portion of the mass of cardiac tissue involved in the fibrillation. Lower energy requirement and fewer shocks to achieve a successful defibrillation are important not only from the standpoints of further size reduction and maintenance of an adequate reserve to increase the interval between defibrillator replacements, but also to avoid potential damage to the heart and skeletal frame of the patient that can occur with frequent or repeated application of high energy shocks.
Under typical defibrillator implant conditions, a coil is introduced into the right ventricle to serve as one electrode or pole, and the defibrillator case (or xe2x80x9ccan,xe2x80x9d as it is often called in the art) that houses the batteries, capacitors, electronic components and circuitry is used as the second pole for the current path during the defibrillation shock. As noted above, the defibrillator case can now be implanted in the pectoral region, usually on the left side, to provide a more effective defibrillation pathway. This is desirable from the standpoint of the implant technique and the cosmetic aspect, but produces an energy and electric field distribution that is not equal, uniform or homogeneous throughout the region of the heart involved in the fibrillation. Measurements performed by the applicants have demonstrated that during application of a shock waveform using standard case, lead and defibrillation coil placements, a field of significantly lower energy (in volts (v) per centimeter (cm), i.e., v/cm) is present at the apex of the left ventricle compared to certain other regions of the heart such as the right ventricular outflow tract. The average electric field strength in the latter region is five to eight times greater than at the apex of the left ventricle.
In practice, then, because a relatively lower energy field is present at some regions that may be critical to defibrillation, the energy gradient sufficient to achieve successful defibrillation by application of the shock waveform mandates an adequate energy level in those regions and, by extension, a considerably higher electric field density in the normally higher energy field locations as well. The result is a further skewing of the inequality or inhomogeneity of the electric field distribution in the strategically important regions.
In one of its aspects, the present invention provides improvements in lead and electrode placements to assist in developing an equal, homogeneous field distribution during application of a defibrillation shock to the heart.
Another problem encountered with present day defibrillators, however, is that despite their capability to provide adequate therapy for sudden electrical instabilities of the cardiac function, they are not similarly capable of providing therapy for an underlying hemodynamically-compromised ventricular function. This means that the patient may suffer an ongoing deficiency in cardiac output, for example, even though the device is effective in correcting isolated events of fibrillation or pacing dysrhythmias.
Clinical investigation performed on patients who suffer from heart failure (i.e., inability of the heart to pump the required amount of blood) indicates that for a certain subset of these patients simultaneous stimulation of the left and right ventricles may be advantageous. In the cardiac cycle, a P wave of the subject""s electrocardiogram (ECG) is produced by a depolarization of the atrial fibers just before they contract, and, when the cardiac impulse reaches the ventricular fibers to stimulate them into depolarization, a QRS complex is produced just before contraction of the ventricular walls. This is followed by a T wave which is indicative of the electrical activity occurring upon repolarization of the ventricular fibers. Simultaneous stimulation of the left and right ventricle would be beneficial therapy to patients whose ECG displays a marked desynchronization in contraction of the two ventricular chambers. In such cases, it is observed that after a right ventricular stimulation, considerable time may elapse for the cardiac impulse to travel from the apex of the right ventricle through the septum and to the free wall of the left ventricle, with the septum contracting earlier than the latter.
Consequently, the mechanical forces of the ventricular contraction are less favorable for an effective hemodynamic output in such patients. The duration or width of the QRS complex may increase because of an injury to the Purkinje fibers that inhabit and stimulate the ventricular septum and the lateral ventricular walls, and which could therefore increase the time for the impulse to spread throughout the ventricular walls. Patients who display a lack of ventricular synchronization primarily exhibit a wide QRS complex indicative of a bundle branch blockxe2x80x94generally a left bundle branch block. Rather than the normal QRS complex width that ranges between 80 to 120 milliseconds (ms), the width of the QRS complex for these patients ranges between 140 and 200 ms.
It is a principal aim of the present invention to provide a method and apparatus for improved hemodynamic performance in patients with heart failure, utilizing an implantable defibrillator of reduced size and increased efficacy which produces a substantially uniform, homogeneous electric field upon application of a defibrillation shock to the heart.
The present invention provides simultaneous pacing of the left and right ventricles of the patient""s heart, and, unlike the prior art in which a pacing lead is inserted into the right ventricle only (through the right atrium and tricuspid valve), a separate pacing lead is also inserted into the left ventricle. Defibrillation coils provided on separate lead wires may be sheathed in respective ones of the two pacing leads for insertion therewith. A defibrillation coil is introduced on a lead into the right ventricle and is seated so that a sensing and stimulating electrode at its distal end resides at the apex of that chamber. An atrial defibrillation coil is inserted on another lead into the right atrium to enable stimulation of the atrial chamber, as appropriate, with pacing pulses and for application of defibrillating shocks. These right chamber (ventricular and atrial) leads enable cardiac pacing stimulation in VVI, VVI-R, DDD, DDD-R, AAI, AAI-R, and other modes, and permit application of defibrillation shocks between the respective coil and the active can or case of the device.
According to the invention, a second ventricular lead is placed in the left ventricle by needle puncture of the arteria subclavia (left subclavian artery) or the brachiocephalic artery, and advancement into the left ventricular chamber through the aortic valve. To avoid interference with the mechanical function of the aortic valve during retrograde passage of this lead into the left ventricle, or thereafter while in place, the lead should be of relatively smaller diameter than traditional ventricular leads (e.g., in a range of 6 to 7 French, or less). Also, its outer surface should be composed of electrically insulative material of very low thrombogenicity (e.g., high performance silicone or polyurethane). Local formation of a thrombus that could embolize and travel to the brain through the aortic valve is additionally avoided by use of platelet (thrombocyte) inhibitors (e.g., preferably ticlopidine, but alternatively or additionally aspirin, GPIIb/IIIa blockers or other inhibitors of the fibrinogen receptor), and by plasmatic coagulation inhibitors (e.g., heparin and hirudin). Such inhibitors are preferably administered for a duration of about one to three months following the surgical procedure. This time period should be adequate to allow a build-up of protective connective tissue around the electrode and also to prevent adhesion of the lead body in the vicinity of the aortic valve.
The left ventricular lead is otherwise of similar construction to the right ventricular lead, and enables pacing stimulation of the left ventricle simultaneously with pacing stimulation of the right ventricle, with resulting improvement in hemodynamics, in large measure by virtue of more organized contraction and avoidance of mitral regurgitation. It is also possible, albeit difficult, to place a lead with a pacing electrode in the left atrium. This is achievable, preferably, by inserting the lead into the left atrium by access from the adjacent distal coronary sinus, or, alternatively, by access through the atrial septum wall from the right atrium. This enables simultaneous pacing of the right and left atria, and synchronization of the simultaneous pacing of the right and left ventricles according to the preset A-V delay, in a DDD mode, plus the capability for bi-ventricular defibrillation.
Most importantly, placement of a defibrillation coil on a lead in the left ventricle allows a considerable reduction of the energy requirement necessary to achieve a successful shock (i.e., termination of ventricular fibrillation, and return to sinus rhythm), with a threshold (DFT) that may be as low as only 2 to 3 joules (J.). To that end, the defibrillation shocks may be applied solely between the two (i.e., right and left) ventricular coil electrodes, and by creating a substantially equal and homogeneous electric field distribution around them through the two ventricular chambers. Alternatively, if individual parameters of thorax geometry and the heart make it necessary or desirable, the shock waveform may be applied between both ventricular coils, operating as one pole, and the device case acting as the other pole. In this alternative configuration, defibrillation is achieved with a somewhat higher DFT than the other, but still considerably below the thresholds seen in the prior art.
Thus, the present invention permits defibrillators with maximum available energy output capability of only 15 joules or less to be implanted with reasonable assurance of successful defibrillation with quite adequate safety margin, even in relatively enlarged hearts. Such low energy requirement means that devices weighing considerably less than 50 grams (g.) and having volumes of less than even 30 cubic centimeters (cc.) can be implemented for this purpose, so that the function generator portion of the device can be sized on the same order as devices that functioned solely as the DDD pacemaker of recent vintage. Hemodynamic improvement attained by simultaneously pacing the left and right ventricles also reduces the occurrence of fibrillation, and thus, the need for defibrillation.
In one aspect of the invention, an implantable medical interventional device is adapted to provide therapy to a patient in whom the device is implanted to treat cardiac dysrhythmias. The device includes function generating means for providing a plurality of functions corresponding to different levels of therapy for treatment of sensed dysrhythmias, including a pulse generator adapted for pacing the patient""s heart by generating stimulating electrical pulses therefor. A pair of thin leads is coupled to the pulse generator and sized for insertion respectively into the left and right ventricles of the patient""s heart when the device is implanted, for application of the stimulating electrical pulses thereto. Each of the leads includes an electrode located on the respective lead for positioning in stimulating relation to cardiac tissue in a respective one of the ventricles and, when energized together by the pulse generator, to simultaneously pace the left and right ventricles.
The lead adapted for insertion into the left ventricle includes an insulative sheath composed of a material of low thrombogenicity, such as a material selected from a group which includes high performance silicone and polyurethane. Also, the insulative sheath is coated at least in part with a biodegradable material to inhibit thrombus formation on the respective lead. The biodegradable material coating may be impregnated with at least one platelet inhibitor (preferably, iloprost) for timed release during disintegration of the coating. The coating may additionally or alternatively be impregnated with a plasmatic coagulation inhibitor such as heparin or hirudin for timed release.
The function generator includes a shock generator adapted for defibrillating the patient""s heart by generating higher voltage electrical shock waveforms. Each of the leads includes a defibrillation coil coupled to the shock generator and located on its lead to be positioned within a respective ventricle to apply an electrical shock waveform to establish a substantially homogeneous electric field of sufficient electrical energy through the ventricles for defibrillation thereof. The defibrillation coils constitute defibrillation poles, and when energized simultaneously the defibrillation coil in the left ventricle is the anode and the defibrillation coil in the right ventricle is the cathode. Alternatively, the ventricular defibrillation coils constitute a single defibrillation pole and are energized simultaneously as an anode, and the metal case within which the function generator is housed constitutes a second defibrillation pole which is energized together with the defibrillation coils as a cathode. An atrial pacing lead includes a pacing electrode coupled to the pulse generator, and a defibrillation coil adapted to be positioned within the right atrium and coupled to the shock generator for defibrillation of the atria.
A variation of the invention is implemented in an implantable pacemaker that includes a pulse generator, a right ventricular pacing lead with an electrode coupled to the pulse generator for positioning in the right ventricle to deliver stimulating pacing pulses from the generator thereto, a left ventricular pacing lead with an electrode coupled to the pulse generator for positioning in the left ventricle to deliver stimulating pacing pulses from the generator thereto, and means for applying selected ones of the stimulating pacing pulses to the right and left ventricular pacing leads for stimulating the ventricles simultaneously. The pacemaker also includes an atrial pacing lead with an electrode coupled to the pulse generator and positioned in the right atrium to deliver stimulating pacing pulses thereto, and means for applying selected ones of the stimulating pacing pulses to the atrial pacing lead, timed for stimulating the atria.
A variation of the invention is implemented in an implantable defibrillator, which includes a shock generator for generating electrical shock waveforms, a right ventricular lead with a defibrillation electrode coupled to the shock generator for positioning in the right ventricle to deliver electrical shock waveforms thereto, a left ventricular lead with a defibrillation electrode coupled to the shock generator for positioning in the left ventricle to deliver electrical shock waveforms thereto, and means for selectively applying electrical shock waveforms of predetermined energy content simultaneously to the right and left ventricular defibrillation electrodes to establish a substantially homogeneous electric field distribution in the ventricles for defibrillation thereof. A right atrial lead of the defibrillator includes a defibrillation electrode coupled to the shock generator for positioning in the right atrium to deliver electrical shock waveforms thereto, and the selectively applying means applies the electrical shock waveforms between the right atrial defibrillation electrode and one of the right ventricular defibrillation electrode, the left ventricular defibrillation electrode, and the metal case of the shock generator, for defibrillation of the atria.
Also according to the invention, a device-implemented method of treating cardiac dysrhythmias is performed with an implantable medical interventional device adapted to deliver therapy to an implant patient. The device possesses the capability to provide pacing therapy to the patient""s heart, and the method includes the steps of sensing a need for ventricular pacing, and responding to the sensed need by delivering pacing pulses simultaneously to both ventricles of the patient""s heart. The device-implemented method also includes the steps of sensing ventricular fibrillation, and responding to the sensed ventricular fibrillation by delivering an electrical shock waveform simultaneously to both ventricles to establish an electric field of relatively uniform distribution and sufficient electrical energy through the ventricles for defibrillation thereof.
Another aspect of the invention resides in a method for providing therapy to a patient from a medical interventional device to treat cardiac dysrhythmias, wherein the device performs a plurality of functions corresponding to different levels of therapy for treatment of sensed dysrhythmias. The device responds to each different type of sensed dysrhythmia to generate an electrical waveform therapy among a variety of therapies appropriate to terminate the respective sensed dysrhythmia, including at least electrical pulse and shock waveform therapies therefor. The method includes implanting an electrical lead with an electrode to deliver at least one of the pulse and shock waveform therapies in each of the right and left ventricles, and electrically connecting each lead to the device to enable its electrode to receive at least one of the therapies.
In the method, prior to implanting the lead, at least a portion of the lead to be implanted in the left ventricle is coated with a biodegradable carrier impregnated with a thrombus inhibitor adapted to be time-released into blood in the locality of the portion of the lead in the left ventricle during disintegration of the carrier, to avoid embolization of a thrombus thereat. The method includes selecting the biodegradable carrier to degrade harmlessly in the blood with negligible systemic impact, and selecting at least one of ticlopidine and aspirin as concomitant oral therapy and intravenous or subcutaneous administration of heparin and hirudin, as the thrombus inhibitor. Also, at least one physical parameter of the biodegradable carrier is selected to fix complete disintegration of the carrier within a period of from about one month to about three months from the time of implanting the lead, for time-release of the thrombus inhibitor over the period. The electrical lead is placed in the left ventricle by puncturing one of the subclavian and brachiocephalic arteries, inserting the lead through the puncture, and advancing the lead through the aortic valve into the left ventricle until the electrode is properly located therein. Each ventricular lead includes both a pacing electrode at the distal end of the lead and a defibrillating electrode proximal of the pacing electrode, and advancement of each lead into the respective ventricle includes placing the pacing electrode in proximity to excitable cardiac tissue of the ventricle.
In the method, the right and left ventricles are paced simultaneously, whereby to reduce the duration of the QRS complex of the patient""s electrocardiogram. Shocks are applied to the defibrillating electrodes of both the right and left ventricular leads simultaneously, whereby to improve uniformity of the electric field distribution through the ventricles to terminate ventricular fibrillation. And shocks are applied to the defibrillating electrodes of both the right atrial lead and the left ventricular lead simultaneously to terminate atrial fibrillation. Also, the local ECG is detected at the left and right ventricles, and the size, morphology and other characteristics thereof are compared to discriminate the origin of the tachycardia, such as to distinguish ventricular and supraventricular tachycardias from one another.